The present invention relates to a device for comparing the value of two resistors as well as to a system for the compensation of integrated resistors whose operating values may vary as a function of their environmental parameters, such as temperature and manufacturing quality.
Integrated resistors on silicon are often required in the field of analog integrated circuits. These resistors may be manufactured by different methods using standard CMOS (Complementary Metal Oxide Semiconductor) technology. However, these integrated resistors have the drawback of being imprecise because of manufacturing variations between semiconductor wafers to be etched, and because of variations in the operating temperature of the circuit. These variations significantly affect the resistance value of the resistors.
In most cases, these integrated resistors cannot be replaced by resistors external to the integrated circuit encapsulation package because of the additional constraints imposed by these external resistors.
To begin with, a first constraint relates to the availability of circuit pins. Planning for a resistor outside the circuit means that one or even two pins of the circuit have to be monopolized for each external resistor. Furthermore, in radio frequency applications, it is not possible to consider implementing resistors external to the package because of the substantial parasitic self-inductive effects that occur at the pins of the package. These radio frequency termination constraints prevent the signal from being output to the external resistor and then from being input again into the package.
In general, the resistors must be placed as close as possible to the circuits that use them in order to minimize the capacitive and self-inductive parasitic effects which prove to be large at the silicon-package connections, and at the package-card connections. In addition, external resistors impose cost constraints.
One prior art approach for the precise adjustment of the value of an integrated resistor relative to its environmental parameters, especially in order to resolve the above-mentioned problem of dispersion due to manufacturing, includes making a resistor and then destroying part of it in an electrical testing environment by sandblasting or by laser, for example, so as to remove resistive material until the desired resistance value of the resistor is obtained.
However, this method of adjusting the value of the integrated resistor, although very precise, is both slow and costly. It is therefore not advantageous in a mass production environment. Furthermore, this approach only partially resolves the problem of achieving perfect control over the value of the integrated resistors used in a given circuit. Indeed, it in no way overcomes the problem of dispersion under temperature that occurs during operation of the circuit. This is another major cause of the variations in the value of the integrated resistors. This approach is therefore not entirely satisfactory in the context of the invention which seeks to obtain very high precision in the use of the integrated circuits.
In view of the foregoing background, an object of the present invention is to enable the precise adjustment of the value of an integrated resistor while mitigating the drawbacks of the prior art by overcoming variations in the manufacturing quality and variations in the temperature during operation of the circuit.
This and other objects, advantages and features according to the present invention are provided by the integration of several compensated resistors. Each compensated resistor comprises a nominal part and several parallel-connected correction resistors (known as compensation resistors). Their use, totally or partially, is as a function of the quality of manufacture of this type of resistor and of the operating temperature of the circuit into which they are integrated. The value of these compensated resistors are adjusted in comparison with a reference resistor external to the circuit.
The choice of the number of compensation resistors to be implemented to attain the desired value is thus constantly re-updated advantageously during the operation of the circuit by a cell integrated into the circuit. This cell is known as a compensation cell and it operates as a resistor comparison device.
The role of this comparison device is to give a digital code to all the compensated integrated resistors. This digital code is the image or reflection of a measurement of comparison between a measurement resistor integrated into the circuit and a reference resistor external to the circuit.
The invention therefore relates to a device for the comparison of two resistors, wherein no voltage reference is implemented and wherein the device uses only analog information carried by currents until they are digitized. The device comprises a measurement circuit that extracts the currents from the two resistors to be compared and copies them to a parallel analog-digital converter. The analog-digital converter performs the division of the extracted currents and converts the ratio of the extracted currents into a digital code. The digital code is the image of the ratio of the two resistors and is constantly re-updated as a function of environmental parameters of the circuit, such as the operating temperature.
The invention also relates to a system for the active correction of the value of compensated resistors integrated into a package based upon an external reference resistor. The values of the compensated resistors may be deliberately different, and moreover, subject to variations caused by the operating temperature and by the quality of manufacture of resistors of this type. Each compensated resistor comprises a nominal resistor and a plurality of parallel-connected correction resistors.
The system implements the comparison device according to the present invention for making a comparison between the value of the reference resistor and the value of a measurement resistor internal to the package. The measurement resistor is designed identically to the nominal part of each of the compensated resistors. The correction resistors are put individually into operation or into the off state as a function of the compensation digital code given by the comparison device so as to obtain the value of the desired compensated resistor.